This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/JP00/02255 which has an International filing date of Apr. 7, 2000, which designated the United States of America.
The present invention relates to a method for the surface modification of a molded plastic, and specifically, it relates to a method for the surface modification which imparts adhesion or adhesive property to coating films, antistatic property and other characteristics to the surface of the molded plastic, as well as to a surface-modified molded plastic obtained by the method. The present invention also relates to a method for modifying a polymer, and specifically it relates to a method for modifying a polymer, which can impart antistatic property and other characteristics to the polymer by the introduction of a polar group, as well as to a modified polymer obtained by the method.
Many of resins for use in optical lenses, optical disks, and other optical materials require adhesion to coating films formed on the surfaces thereof, in addition to a high transparency, a low birefringence, and other basic characteristics. Add to this, demands are made on these resins to minimize changes due to environmental conditions in optical characteristics (especially, refractive index) and in dimension. Poly(methyl methacrylate) (PMMA), a typical transparent resin, is excellent, for example, in adhesive property and adhesion to coating films, but highly absorbs moisture and therefore largely changes in refractive index, which causes changes in focal distance. Accordingly, PMMA cannot be used as optical lenses in, for example, cameras. In addition, PMMA greatly changes in dimension due to moisture absorption, and when PMMA is applied to magneto-optic disks and other applications in which an inorganic film is applied thereon, the inorganic film cracks with dimensional change. A polycarbonate (PC) predominantly used for optical disks is relatively free from the above problems but exhibits a high birefringence.
On the other hand, to solve the problems due to moisture absorption, polyolefin resins each having a non-aromatic ring introduced into a principle chain (trade names: APO and ZEONEX, etc.) have been reported as optical material polymers in recent years. However, this resin has a high hydrophobicity which is a feature of polyolefin polymers, and therefore has a problem of a low adhesion to coating films formed on the surface. In order to solve this problem, a polyolefin resin (trade name: ARTON) having a polar group introduced into the non-aromatic ring is on the market. However, this resin inevitably has a higher hygroscopicity than a polyolefin resin having no polar group.
As is described above, the low water absorption which is required for ensuring optical stability and dimensional stability, and the hydrophilicity which contributes to adhesion to coating films are in trade-off relationship. To make these two characteristics compatible with each other, it is ideal that the inside of a molded article such as a lens or disk substrate is made of a hydrophobic polymer having a low hygroscopicity and the surface thereof alone is made of a hydrophilic polymer having a satisfactory adhesion to coating films.
Generally, known methods for the surface modification of plastics are, for example, (i) a method of coating the surface with a surfactant, (ii) a method of activating the surface through corona discharge, and (iii) a method of activating the surface though laser abrasion. However, the method (i) is insufficient in adhesion to coating films, although it is useful, for example, for imparting a temporary antistatic property. The methods (ii) and (iii) invite fine projections and depressions on the surfaces of plastics.
On the other hand, many of polymers for general use such as polyethylenes, polypropylenes, and polystyrenes have no free polar group or reactive functional group in a molecule and have a strongly hydrophobic characteristic, and are therefore generally static-prone. Accordingly, these polymers are usually incorporated with antistatic agents prior to use. However, this technique often invites a problem of bleeding out of the antistatic agents. The polymer itself must have a polar group to avoid this problem.
Generally, known methods for producing a polymer having a polar group include, for example, (i) a method of copolymerizing a material with a monomer having a polar group in a polymerization step, (ii) a method of converting an inherently hydrophilic polymer such as cellulose acetate into a hydrophobic polymer and then controlling the degree of esterification thereof through deesterification (saponification), (iii) a method of converting an aromatic ring of a polymer having the aromatic ring such as benzene ring into a functional group, and (iv) a method of combining a hydrophilic group such as carboxyl group with an unsaturated moiety of an aliphatic cyclic hydrocarbon polymer having the unsaturated-bond moiety such as dicyclopentadiene.
However, these methods are much limited in their application range. For example, a special monomer is required for converting an aromatic ring into a functional group, or the method can be applied only to a specific polymer previously having a functional group such as a hydroxyl group or an unsaturated moiety.
As a possible solution to the problems, there is a method of introducing a polar group into a principle chain through oxidization of, for example, a polypropylene. However, according to conventional oxidation techniques, the principle chain is liable to cleave to thereby yield a low molecular weight product.
Accordingly, it is an object of the present invention to provide a method for the surface modification of a molded plastic, which can impart hydrophilicity only to the surface without significantly varying the water absorption of the molded plastic.
It is another object of the present invention to provide a method for modifying a surface of a plastic, which can hydrophilize only the surface of a molded article made of a hydrophobic polymer to thereby improve adhesion to coating films, adhesive property, and antistatic property.
A further object of the present invention is to provide a method for the surface modification of a molded plastic, which can hydrophilize the surface without causing projections and depressions on the surface.
Yet another object of the present invention is to provide a surface-modified molded plastic which is satisfactory in optical and dimensional stability and has excellent adhesion to coating films, adhesive property and a high antistatic property.
A still further object of the present invention is to provide a versatile method of modifying a polymer, which can easily introduce a polar group into a polymer, especially to a hydrophobic polymer.
It is another object of the present invention to provide a method for modifying a polymer, which can introduce a polar group into the polymer without cleaving a principle chain.
It is still another object of the present invention to provide a method for modifying a polymer, which can introduce a desired polar group into the polymer in a desired proportion.
A yet further object of the present invention is to provide a polymer having a satisfactory antistatic property.
After intensive investigations to achieve the above objects, the present inventors found that the treatment of a molded plastic with an oxygen-atom-containing gas in the presence of a specific catalyst can efficiently hydrophilize only the surface of the molded plastic, and that the treatment of a polymer with an oxygen-atom-containing gas in the presence of a specific catalyst can efficiently introduce a polar group into the polymer. The present invention has been accomplished based on these findings.
Specifically, the present invention provides a method for the surface modification of a molded plastic. The method includes the step of treating a molded plastic with an oxygen-atom-containing gas in the presence of an imide compound represented by the following formula (1): 
(wherein R1 and R2 are each, identical to or different from each other, a hydrogen atom, a halogen atom, an alkyl group, an aryl group, a cycloalkyl group, a hydroxyl group, an alkoxy group, a carboxyl group, an alkoxycarbonyl group, or an acyl group, or R1 and R2 may be combined to form a double bond or an aromatic or non-aromatic ring; X is an oxygen atom or a hydroxyl group; and one or two of N-substituted cyclic imido group indicated in the formula (1) may be further formed on the R1, R2, or on the double bond or aromatic or non-aromatic ring formed by R1 and R2).
As the oxygen-atom-containing gas, at least one gas selected from oxygen, carbon monoxide, nitrogen oxides, and sulfur oxides can be used.
The present invention provides, in another aspect, a surface-modified molded plastic obtained by treating a molded plastic according to the aforementioned method.
In a further aspect, the present invention provides a method for modifying a polymer, which includes the step of treating a polymer with a oxygen-atom-containing gas in the presence of the imide compound represented by the formula (1).
At least one gas selected from oxygen, carbon monoxide, nitrogen oxides, and sulfur oxides can be used as the oxygen-atom-containing gas.
In yet another aspect, the present invention provides a modified polymer obtained by treating a polymer according to the aforementioned method.